The present invention relates to a high-pressure sodium vapour lamp, comprising a hermetically closed tube made of translucent material of high thermal resistivity, particularly of ceramics, at least two electrodes arranged in said tube for initiating a discharge arc, and a filling containing noble gas, particularly xenon and metal additives as sodium and mercury and/or cadmium, wherein the metal additives are present in a quantity as to ensure pressure of saturated vapour at the temperature of operating of the lamp.
In the high-pressure sodium vapour lamps the discharge is usually initiated in a tube made of translucent material, for example, ceramics, such as alumina of high thermal resistivity. This tube is usually arranged in an outer vessel including also the electric input means of the tube. The outer vessel is evacuated to vacuum. The current is supplied to the electrodes arranged in the tube, on the two ends thereof by means of electric inlets made of niobium or of metallized ceramics closing elements or of metallic ceramics. The tube comprises a filling containing noble gas (or some noble gases) and metal additives. Noble gas is important for initiating the arc discharge and for this purpose xenon seems to be the most advantageous. As metal additives sodium and mercury and/or cadmium should be used. The character of the discharge is determined mainly by the sodium which is present in a working lamp under pressure of about 0.1 to 1.0 bar range, while the other metal additives are intended to adjust the electric parameters of the lamp.
During operation the high-pressure sodium vapour lamp is gradually losing its sodium content. Therefore the quantity of metal additives is chosen to be so high that decreasing quantity of sodium do not cause higher change in the composition of the fluid phase of the metal additives. For this purpose a high quantity of the metal additives is required and, in this case, the quantity of the metal additives being present in fluid phase is many times higher than that of the metal additives being present in vapour phase. A sodium vapour lamp based on this principle is shown in U.S. Pat. No. 3,384,798. The common disadvantage of the high-pressure sodium vapour lamp described in that patent or one of similar construction is that decreasing quantity of sodium and gradual blackening of the ends of the ceramic tube result in increasing burning or operating voltage of the sodium lamp. The operating voltage can reach such a high value at which the oscillation of the supply voltage will result in extinction of the lamp. In order to prevent this extinction after a possible long life time, the sodium vapour lamps are usually adjusted to be operated at relatively low supply voltage, which causes the disadvantage that at the beginning period of the use of the high-pressure vapour sodium lamps can not take up the nominal power and therefore they can not give the desired light output.
Another solution is described in U.S. Pat. No 4,075,530, according to which the high-pressure sodium vapour lamp is filled with nonsaturated vapour, which means that during the operation of the lamp the entire quantity of sodium and mercury is in vapour phase. In the lamps built up according to this solution, however, the operating voltage of the sodium vapour lamp is not changing when supply voltage is changing, but a low quantity of metal additives is filled in therefore the sodium is quickly decreases, which leads to the fact that the lamps originally intended to operate as a sodium vapour lamp, will operate as a mercury lamp.